RECENT researches have shown that gases are rendered conductors of electricity when negatively-charged ions move through them with a high velocity. Thus the kathode rays and the Lenard rays possess the property of ionising gases through which they pass (J. J. Thomson, “The Discharge of Electricity through Gases”). Becquerel (Comptes rendus, March 26, 1900) also has recently shown that the conductivity produced by radium is due to small negatively-charged particles given off by the radio-active substance. In these cases the charged particles which ionise the gas move with velocities nearly equal to the velocity of light.